The present invention relates to solid oxide fuel cells, particularly to thin film solid oxide fuel cells, and more particularly to the fabrication of thin film solid oxide fuel cells using vapor deposition techniques to miniaturize the assembly thereof for the formation of single and multiple cells.
Fuel cells are electrochemical devices that convert the chemical energy in hydrogen or carbon monoxide and oxygen (in air) to electricity. A solid oxide fuel cell (SOFC) consists of three basic components: an electrolyte separating an anode and cathode. A thin film solid oxide fuel cell (TFSOFC) offers improvements in cost, reliability, efficiency, power density and specific power over other fuel cells.
An option to eliminate emissions from vehicles, reducing dependence on oil and enabling the use of alternative fuels, is the development and commercialization of fuel cells that are cost effective, safe, and reliable. The maturation of vacuum coating technology in the microelectronics and photovoltaics industries has enabled the potential manufacture of thin film solid fuel cells (TFSOFCs) at costs less than any other current fuel cell design and possibly as low as the unit cost of internal combustion engines. Whereas current SOFC designs, based on state-of-the-art ceramic powder engineering require a 1000.degree. C. operation, TFSOFCs can operate at temperatures less than 750.degree. C. with a dramatically reduced volume and mass for a given power output.
The reduction in operation temperature enables the alternative selection of materials for the greatly reduced dimensions of thin films as opposed to the SOFCs synthesized with bulk ceramic methods. Herein lie the difficulties encountered in TFSOFC synthesis and original solutions to the following problems.
The electrolyte must have sufficient mechanical integrity to survive fabrication and operational environments. The electrolyte must be a thermodynamically stable oxide layer on the order of 1-10 .mu.m thick with a good oxygen ion transference number, low electron conductivity, and high enough density to prevent the fuel and air mixing. Yttria stabilized zirconia (YSZ) and cerium oxide (CeO.sub.2), for example, are candidate materials, but they have not been synthesized in sub 10 .mu.m thick, defect-free layers. Essentially, any other ion conducting oxides can be used.
The electrodes must be mechanically strong with a coefficient of thermal expansion (CTE) that matches the electrolyte and must be electrically conductive, yet sufficiently porous to allow for gas flow there through. Prior attempts to vapor synthesize a metal doped, YSZ layer which is porous have been unsuccessful. In addition, the electrolyte-electrode interface must be structurally stable and supply sufficient line contact for the dissociationrecombination reactions as well as ionizations and ion penetrationsextractions.